It's the terminology that throws people off. A steam iron is an IRON with vents to release steam. A steamer is what you're referring to. They are available in a travel size - Oreck makes a good one.

IRONS of any kind (that press clothes) are simply not allowed on cruise ships, for many reasons; one being they get so hot they could overload circuits. Any good steward will remove an iron from a cabin if he/she sees one.

All the Carnival ships I have been on have passenger laundry rooms on each deck, complete with washers, dryers, ironing boards, and irons.